[mypy] Fix type annotations for linked_list/__init__.py

[archaengel]

Before:

$ mypy --ignore-missing-imports data_structures/linked_list/__init__.py
__init__.py:24: error: Incompatible types in assignment (expression has type "Node", variable has type "None")
__init__.py:31: error: "None" has no attribute "item"
__init__.py:32: error: "None" has no attribute "next"
__init__.py:53: error: Need type annotation for "item_list" (hint: "item_list: List[<type>] = ...")
Found 4 errors in 1 file (checked 1 source file)

After:

$ mypy --ignore-missing-imports data_structures/linked_list/__init__.py
Success: no issues found in 1 source file

$ python3 -m doctest -v data_structures/linked_list/__init__.py
...
15 tests in 10 items.
15 passed and 0 failed.
Test passed.

Related issue: #4052

Describe your change:

Fix type annotations for linked_list/__init__.py.

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• Documentation change?

Checklist:

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized.
• I know that pull requests will not be merged if they fail the automated tests.
• This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
• All new Python files are placed inside an existing directory.
• All filenames are in all lowercase characters with no spaces or dashes.
• All functions and variable names follow Python naming conventions.
• All function parameters and return values are annotated with Python type hints.
• All functions have doctests that pass the automated testing.
• All new algorithms have a URL in its comments that points to Wikipedia or other similar explanation.
• If this pull request resolves one or more open issues then the commit message contains Fixes: #{$ISSUE_NO}.

[archaengel]

#4927 fixes errors in this directory